This invention relates to a method of reading the radiation image recorded two-dimensionally in an imaging plate that uses a stimulable phosphor as a radiation detecting medium. The invention relates particularly to a method characterized by rapid reading of the radiation image in the imaging plate and its continuous reading as the plate is illuminated with radiation. Because of these features, the invention is useful in understanding dynamic events based on rapid processing and real-time radiation image detection as in medical X-ray diagnosis, X-ray structural analysis and research, neutral structural analysis and research, and autoradiography using X-rays and neutrons.
FIG. 1 shows a conventional method of reading the radiation image recorded two-dimensionally in an imaging plate that uses a stimulable phosphor as a radiation detecting medium. An irradiated imaging plate 201 on a transport belt 202 is moved as it is scanned over the surface with a spot beam of exciting light 204 from a light source 203 that has been reflected by a galvano-mirror 205. The exciting light is typically laser light. The exciting light 204 generates photostimulated fluorescence 206 that is directed by a condensing guide 207 typically made of an optical fiber bundle and passed through a bandpass optical filter 208 having the wavelength of the photostimulated fluorescence as a center wavelength before it is detected with a photomultiplier tube 209. The detected light is then supplied to a signal processor 210, where it is converted to digital signals in accordance with intensity and reconstructed as a radiation image; this is the process of reading the radiation image recorded two-dimensionally in the imaging plate 201 [Nucl. Instr. and Meth., A246, p. 572-578 (1986); Gendai Kagaku, No. 223, p. 29-36 (1989)].
Another method currently used to transport the imaging plate is by rotating it on a drum [Nucl. Instr. and Meth., A310, p. 366-368 (1991); Hoshasen, Vol. 23, No. 2, p. 43-51 (1995)].
In the conventional method of reading radiation images, an imaging plate mounted on a belt or a drum is moved as its surface is scanned two-dimensionally pixel by pixel with a spot beam of laser light. Reading the radiation image from the entire surface of the imaging plate by this method is a time-consuming job. In addition, the imaging plate is moved mechanically in order to read one of the two axes in two-dimensional scanning and this not only limits the moving speed but also increases the chance of mechanical troubles. As a result, it has been difficult to perform real-time detection of the radiation image at high speed as the imaging plate is illuminated with radiation.
As a further problem, detecting the photostimulated fluorescence from the same side of the imaging plate as where the exciting light is incident makes the system highly sensitive to the intense scattering of the exciting light which can be background noise to the radiation image. In order to avoid the undesired effects of the scattered light, improvements have been made on the technique of processing the surface of the imaging plate and the method of condensing the photostimulated fluorescence by the condensing guide.
The principal object of the invention is to provide a method of reading the radiation image in an imaging plate that is capable of rapid reading of the radiation image with high sensitivity and reduced background noise from the scattering of exciting light, that minimizes the mechanically moving parts to reduce the causes of trouble, and that enables the radiation image to be read continuously in real time and stored as a digital image while the imaging plate is illuminated with radiation.
In order to attain this object, the exciting light for reading the photostimulated fluorescence from color centers created by radiation in the bulk of the stimulable phosphor used as a detecting medium in the imaging plate is applied in a rectangular shape to the front side of the imaging plate whereas the photostimulated fluorescence from the back side of the imaging plate is detected with a planar array of wavelength shifters that is positioned at right angles to the rectangular shape of exciting light, thereby reading the two-dimensionally recorded radiation image. This enables the radiation image to be read from the imaging plate at high speed since the whole image can be read by scanning with the moving rectangular shape of exciting light. As a further advantage, the imaging plate fixed on the scan table is scanned by illumination with the rectangular shape of exciting light and this reduces the causes of trouble since the only mechanical part of the system is found in the mechanism for performing one-directional scan.
The imaging plate used in the method of the invention which relies upon a stimulable phosphor as a detection medium is capable of transmitting light through both front and back sides and exciting light is incident on the front side whereas the photostimulated fluorescence emitted from the back side is detected. Therefore, almost all of the scattered exciting light that is emitted simultaneously with the photostimulated fluorescence is rejected by the bandpass optical filter that is placed behind the imaging plate and which has the wavelength of the photostimulated fluorescence as a center wavelength. Any leakage of the scattered exciting light from the optical filter is incident on the wavelength shifters in fiber form at right angles and only a very portion of it propagates through the shifters. The fluorescence shifted in wavelength by the wavelength shifters is passed through a bandpass optical filter having the wavelength of the fluorescence as a central wavelength and this ensures that the scattered exciting light will not be launched into a photodetector to become background noise to the radiation image.
The imaging plate used in the invention may be so modified as to comprise a planar array of fibers made of a transparent glass capable of emitting photostimulated fluorescence. Since this type of glass itself is in fiber form, the imaging plate allows for efficient reading of photostimulated fluorescence from the color centers created by radiation. The already described method of reading can be employed without any change and the radiation image in the imaging plate can be read at high speed and with high sensitivity by scanning with the moving rectangular shape of exciting light.
In the conventional method, a photomultiplier tube has been used as a photodetector. In the invention, a multi-channel photodetector is substituted that consists of optics, a streak tube and a CCD camera. The streak tube consists of a photocathode, electrodes with a slit, a deflector, a micro-plate channel and a fluorescent screen. The fluorescence emitted from the array of wavelength shifters is passed through the optics and allowed to be incident on the horizontal axis of the streak tube. Since the streak tube sweeps the deflector by time, its vertical axis is swept in correspondence with the reading position in the longitudinal direction of the imaging plate, whereby a streak image is produced on the fluorescent screen. The streak image is cumulatively detected with the CCD camera capable of two-dimensional recording and the accumulated signals are digitized with a signal processor, thereby reading the radiation image recorded two-dimensionally in the imaging plate. Using this multi-channel photodetector, one can detect photostimulated fluorescence, integrate the amount of fluorescence and store a large volume of data within very short times. On the other hand, if photomultiplier tubes are to be used, their number must be equal to that of the wavelength shifters used and the same numbers of signal amplifiers, integrators and analog/digital converters must also be used at subsequent stages; this raises the need to use a huge volume of modules but offers the advantage of enabling real-time processing. It should also be noted that the time required for the analog/digital converters to digitize signals is xe2x80x9cdead timexe2x80x9d, which is a dominant factor in determining the scanning time.
The present inventors also developed a method of condensing and detecting photostimulated fluorescence via optical fibers as a means for effective use of the multi-channel photodetector composed of optics, a streak tube and a CCD camera. This method uses an imaging plate reading apparatus comprising an imaging plate using a stimulable phosphor as a detecting medium, an exciting light source that emits a wavelength of light capable of exciting the stimulable phosphor, a mechanism by which the exciting light outputted from the exciting light source is applied in a rectangular shape, a single array of optical fibers into which the rectangular shape of photostimulated fluorescence produced by illumination with the exciting light is launched from an end to be condensed, a scan mechanism in which both the mechanism for applying the exciting light in a rectangular shape and the optical fiber array are mounted on the moving section and which moves them simultaneously to scan the entire surface of the imaging plate, a bandpass optical filter having the wavelength of the photostimulated fluorescence as a central wavelength, a multi-channel photodetector for detecting in multiple channels the photostimulated fluorescence outputted from the optical fiber array and which consists of optics, a streak tube and a CCD camera in combination, and a signal processor for processing the detection signals from the multiple channels. In this apparatus, the photostimulated fluorescence condensed by the optical fiber array is passed through the bandpass optical filter having the wavelength of the photostimulated fluorescence as a central wavelength. The photostimulated fluorescence is then passed through the optics and allowed to be incident on the horizontal axis of the streak tube whereas the vertical axis of the streak tube which sweeps by time is swept in correspondence with the reading position in the longitudinal direction of the imaging plate, whereby a streak image is produced on the fluorescent screen of the streak tube. The streak image is cumulatively detected with the CCD camera and the accumulated signals are read, digitized and reconstructed as a radiation image with the signal processor, whereby the radiation image recorded two-dimensionally in the imaging plate can be read.
Thus, it has become possible to perform a rapid process comprising the steps of detecting radiation image signals from the imaging plate, accumulating them, converting them to digital signals, and storing the digital signals in the data storage unit in the signal processor. By synchronizing the step of scanning the rectangular shape of exciting light continuously and repeatedly over the imaging plate from the start to the end point in its longitudinal width with the process comprising the steps of detecting radiation image signals from the imaging plate, accumulating them, converting them to digital signals and storing the digital signals in the storage unit in the signal processor, the two-dimensional radiation image can be continuously read from the imaging plate as it is illuminated with radiation.
However, the use of the multi-channel photodetector composed of optics, a streak tube and a CCD camera has a difficulty in that the process of scanning with exciting light must be suspended until all data accumulated in the CCD camera is digitized and incorporated into the signal processor. To solve this problem, the streak image on the fluorescent screen of the streak tube is detected and accumulated in two or more CCD cameras that are selectively operated in synchronism with the scan of the imaging plate so that while one CCD camera is in the process of detection and accumulation, the radiation image data already accumulated in another CCD camera is read out. With this approach, there is no need to suspend the scanning operation while the accumulated radiation image data is being read out of the CCD cameras and two-dimensional radiation image can be read continuously from the imaging plate as it is illuminated with radiation.
To operate two or more CCD cameras selectively, they are mounted at equal angles on a rotating table and moved in synchronism with repeated scan of the imaging plate with the rectangular shape of exciting light until one CCD camera comes into registry with the streak image on the fluorescent screen of the streak tube. While this CCD camera is in the process of detection and accumulation, the radiation image data already accumulated in another CCD camera is read out. With this approach, there is no need to suspend the scanning operation while the accumulated radiation image data is being read out of the CCD cameras and two-dimensional radiation image can be read continuously from the imaging plate as it is illuminated with radiation.
In the foregoing description, the radiations to be measured are limited to ionizing radiations such as X-rays, xcex3-rays, xcex2-rays, xcex1-rays and particle rays that can be directly measured with the imaging plate, but these are not the sole examples of the invention. If at least one neutron converter selected from among Gd, 6Li and 10B which can convert neutrons to an ionizable radiation is mixed or combined with the radiation detecting stimulable phosphor in the imaging plate, the resulting imaging plate can be used to detect neutrons and hence is suitable for reading two-dimensional neutron images. By continuously scanning this imaging plate for neutron detection, two-dimensional neutron images can be continuously read from the imaging plate as it is illuminated with neutrons.